A commonly used resource for outdoor navigation is satellite positioning technology, otherwise known as a Global Navigation Satellite System (GNSS). One example of a fully operational GNSS is the United States NAVSTAR Global Positioning System (GPS)—which will be referred to below when generally discussing satellite positioning technology. However, it will be appreciated that satellite positioning technologies other than GPS may be used in its place.
The operation of GPS is well known in the art, and generally employs a GPS receiver arranged to receive signals from a number of GPS satellites. Each satellite broadcasts its own location and providing the GPS receiver can receive the broadcasted signals from a sufficient number and distribution of satellites, the GPS receiver can infer its own position.
An entity wanting to self-localise may therefore employ a positioning system having a GPS receiver. However, in the event that a GPS receiver is not able to infer its position—for example due to signal interference, then it may be possible for the positioning system to make use of other positioning resources.
An Inertial Measurement Unit (IMU) is one example of such a positioning resource. An IMU can be used to perform a technique known as dead reckoning to track the relative movement of an entity from a known start point. As a further example, if a positioning system is being used in a car, and the car enters a tunnel (at which point the GPS signals are lost), information from the car's odometer can be used to track the distance travelled through the tunnel. Additionally, sensors of an inertial measurement unit such as attitude sensors (e.g. digital compass), gyroscopes and the like can be used to provide additional information to the positioning system as to the likely position relative to the known start point.
However, without an external point of reference, the errors caused by the inaccuracies of the sensors build up over time. Whilst very accurate sensors can be used to determine position to a high degree of accuracy, such solutions can be expensive and heavy, and in any case, all positioning systems relying solely on the technique of dead reckoning suffer from drift. An Inertial Navigation System (INS) combines data from an IMU with regular updates from a GPS receiver in order to constrain this drift and provide accurate navigation information at a high update rate.
Another type of positioning resource that can be used is a radio signal positioning system which is used to navigate via radio signals other than those transmitted by GPS satellites—for example, using radio signals transmitted by cellular telephone base stations, television and radio transmitters and the like. The signals transmitted by these transmitters generally have distinguishing characteristics that can be exploited by a suitable positioning system for navigation. In particular, the signals transmitted may contain unique code words and this together with information about the location of each of these transmitters can be used to determine the position of an entity using known localisation techniques such as multilateration and Enhanced Observed Time Difference (EOTD).
However, a significant problem is that the positions of the transmitters need to be known by the radio positioning system for each transmitter to be successfully used as a point of reference during navigation. This information can be stored in advance by the radio positioning system or broadcasted by the transmitter itself. However, when locations of the radio transmitters are not known or at least are not known to a sufficient degree of accuracy, the radio transmitter cannot be used for navigation.
It is an object of the present invention to alleviate these problems, at least in part.